Composite materials, such as carbon-carbon, offer advantages of light weight and good mechanical properties for a variety of aerospace and other applications, such as brake pads and uncooled engine and other airplane parts. One of the most common fabrication methods of such composite structures involves densification of a porous body having the approximate desired shape by means of chemical vapor deposition (CVD) and infiltration (CVI). This method involves flowing a stream of vapor containing the desired element or compound over and around the part to be densified, while that part is kept at a temperature sufficient to decompose the precursor vapor. Under the appropriate conditions, the precursor decomposes in such a way as to produce the desired element or compound within the pores of the part, thus increasing its density.
The densification rate usually increases with increasing precursor partial pressure and increasing substrate temperature. For practical reasons, it is desirable to reduce the processing time, or equivalently increase the densification rate, as much as possible. On the other hand, the microscopic structure and corresponding macroscopic properties of the densified material, such as friction and wear rate, vary with deposition temperature and other conditions. Furthermore, increasing the pressure and/or temperature may produce undesirable homogeneous nucleation of powders in the gas phase (soot formation in the case of carbon compounds) instead of inside the pores of the substrate, leading to closure of the surface pores, and thus hindering further densification.
A common application of CVD/CVI involves densification of porous carbon substrates. Typically, a large number of such substrates are placed in an enclosure uniformly heated to a temperature of about 1000.degree. C. and exposed to a stream of precursor vapor, e.g. methane. This approach is known as hot-wall CVD, since the walls of the reaction vessel are kept at or slightly above the substrate temperature. In this procedure, each substrate is at a uniform temperature throughout its volume. This procedure has the major drawback of extremely long cycle time, of the order of 600-1200 h to achieve desired degree of densification. The main reason for this is that the pressure and temperature inside the reactor must be kept relatively low, in order to produce the desired carbon microstructure, e.g. so-called rough laminar or smooth laminar and to prevent homogeneous nucleation in the gas phase with soot formation. Furthermore, the process usually must be interrupted at least once, and often several times, to permit grinding of the substrate exterior surfaces in order to open clogged surface pores. Without intermediate grinding operation, the desired density cannot be achieved. Also, it is not possible to measure the end-point of the reaction and the process is stopped after a pre-set time. Thus, substrates may not be completely or optimally densified and the time at temperature may be unnecessarily lengthened, resulting in wasted resources and added cost.
It has already been proposed to reduce the CVD cycle time, utilizing approaches termed "thermal-gradient CVI".
Kotlensky (Proc. 16th National Symposium of the Society of Aerospace, Material and Process Engineers, Anaheim, Calif. Apr. 1971 pp. 257-265.14)) describes a thermal-gradient CVI process in which a porous carbon-carbon substrate is placed on a side of a hexagonal graphite mandrel (receptor) which is heated by an induction coil driven at 3 kHz. The substrate does not have circumferential continuity around the mandrel and thus cannot couple to the coil. The porous carbon substrate also has a much lower electrical conductivity than the mandrel. The substrate is heated by thermal radiation from the mandrel. Due to the very low thermal conductivity of the porous carbon substrate and the radiation heat loss from the opposite surface of the substrate to the water-cooled coil and water-cooled chamber walls, that opposite surface is at a much lower temperature. Thus, a significant thermal gradient is established across the substrate, which enables heating the hot surface to higher temperatures than used in isothermal CVI, without immediate pore closure. Carbon is deposited first (mainly) in the hottest portion of the substrate. As that portion of the substrate becomes denser, its thermal conductivity increases somewhat, causing a decrease in the temperature gradient. Thus the temperature increases somewhat with time through the substrate, from the surface adjacent to the mandrel towards the opposite surface. However, since the heating of the substrate still depends primarily on heat radiation from the mandrel, the temperature in those portions of the substrate further from the mandrel is lower than that in the region adjacent to the mandrel. Thus, although the local, microscopic deposition rate is higher initially in the hot region adjacent to the mandrel, the overall process is still relatively slow. Additionally, since this process is run at a pressure of 1 atm (760 Torr at sea level), substantial amounts of undesirable soot and tar are formed by homogeneous nucleation in the gas phase, even inside the pores of the substrate, resulting in a material which has lower thermal and electrical conductivity, which is less graphitizable and has degraded mechanical properties.
Lieberman and Noles (Proc. 4th Int. Conf. on Chemical Vapor Deposition, Boston, Mass., Oct. 1973 pp. 19-29), Stoller et al. (Proc. 1971 Fall Meeting of the Metallurgical Soc. of AIME, Detroit, Mich., published in Weeton and Scala, Eds. "Composites: State of the Art", Met. Soc. of AIME [1974]pp. 69-136), and Lieberman et al. (J. Composite Materials 9 337-346 [1975])) describe a thermal-gradient CVI process, in which a porous carbon-carbon felt is mounted around a truncated-conical graphite susceptor which is heated by a conical-shaped induction coil. The felt, which is initially flat, is made into a truncated hollow cone by sewing with a nylon thread along the entire length of the cone. Since nylon is electrically insulating, there is no induced electrical current around the circumference of the felt and the felt does not couple electrically to the coil. Stoller et at. state that a reduction of a factor of two only is achieved in cycle time, compared to the isothermal process, and that the technique is limited to densifying one item at a time. This process also suffers from soot formation inside the pores of the felt-substrate, due to the very high temperature and high pressure employed. The rate of densification is again not much faster than achieved in the isothermal process because the heating of the felt depends primarily on thermal radiation from the mandrel, just as in Kotlensky's process.
Houdayer et al. in U.S Pat. 4,472,454 describe a method for rapid densification of porous annular carbon parts by placing such parts around a cylindrical conducting susceptor, immersing this assembly in a liquid precursor, preferably cyclohexane, and heating the parts by induction to a temperature sufficient to cause the liquid to vaporize and deposit carbon inside the pores of the parts. These authors claim that this carbon densification process takes approximately 1/100 of the time to complete compared to densification from the vapor phase by prior art and that the parts densified by their process have a texture and physical characteristics identical to those obtained accordinc to the prior art. This approach requires immersion in a liquid precursor, which is limiting for practical applications. The large thermal gradient imposed on the substrate by heat loss through the boiling precursor liquid prevents the outer portions of the substrate from ever attaining a temperature sufficiently high to produce CVD carbon. This results in the outer portions of the substrate having significantly lower density than the inner portions, requiring corresponding oversizing of the substrate. Since the central susceptor is the sole heat source, the quality and microstructure of the carbon deposited in the substrate along the radial direction is not constant. For example, scale-up of this process to densify a 21 inch (53.3 cm) outer diameter substrate appears very difficult, because of the large power supplies and cooling facilities required.
Finally, Bristow and Hill (Inst. Chem. Eng., London, England, Symp. Set. 43 [1975]pp. 5--5 through 5-11) teach against using an induction coil to densify porous carbon-carbon composites by thermal-gradient chemical vapor deposition. They claim that an induction heating arrangement may cause the process to run out of control. They describe a process using a central resistance heater and do not mention any advantage in reduced cycle time.